(1) Field of the Invention
This invention relates to variable speed drives, and more particularly to variable speed drives controlling compressor motors.
(2) Description of the Related Art
Variable speed drives (VSDs) are commonly used to power electric motors of compressors. An exemplary application is refrigeration compressors used to cool water for building climate control, industrial cooling or the like. The electricity consumption of such VSDs may represent a substantial portion of the energy consumption of the building.
It is known to control the VSD (e.g., a VSD having an active rectifier front end) to compensate for losses associated with the reactive power of the motor being powered. The reactive power (measured in volt ampere reactive or VAR) is energy used by the motor to build up its magnetic field. VARs are not converted into work and thus represent losses. The VSD may be controlled to ideally zero the reactive power of the combined VSD and motor system. So controlled, the ratio of real power to apparent power of the system (the xe2x80x9cpower factorxe2x80x9d) will be one.
The apparent power (measured in volt ampere or VA) is the mathematical product of voltage and current in an AC system. Because voltage and current may not be in phase in an AC system, the apparent power may exceed the real power. Reactive loads (inductance and/or capacitance) in an AC system will cause the apparent power to exceed the real power. Apparent power may be graphically represented in vector form as the hypotenuse of a right triangle whose other sides are real power and reactive power.
Accordingly, certain aspects of the invention relate to the operation of a VSD to power a first load. The VSD is powered by a power source simultaneously powering a second load. An AC reactive power is monitored which may comprise a second AC reactive power of the second load. Responsive to the monitored reactive power, the VSD is controlled to maintain a first AC reactive power of the VSD and first load opposite the second AC reactive power.
In various implementations, the magnitude of the first AC reactive power may be maintained to at least 20% of a magnitude of the second AC reactive power. The magnitude of the first AC reactive power may be maintained to at least the lesser of: 50% of the magnitude of the second AC reactive power; and 20% of a real power of the variable speed drive.
Other aspects of the invention are directed to a VSD system having means for monitoring the monitored AC reactive power and means for controlling the VSD responsive to the monitored AC reactive power.
The controlling may maintain a combination of the AC reactive power of the variable speed drive and compressor motor and the AC reactive power of the additional load to no more than 50% of a wattage of the combination.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.